This thread is dedicated to the "splashing" of other classes on a primarily Monk build. Just to make things clear(er), I'm going to arbitrarily say that in order to qualify for this, the build must be at least half Monk levels by the time you hit level 6 and forever there after. I'm trying to focus on what other classes have to offer a Monk build, not what the Monk class has to offer other builds.
Of course, at the time of this posting being a couple weeks before "Monk week", we have to operate on some assumptions about what the Monk class will have. I'm going off my 3.5 Player's Handbook for now as it's the best source available. That said, what I'd like to do here is go over every class that you could multiclass with a Monk. I want to start with a review of what the Monk does get in PnP so that we know where we're coming from.
Monk level - benefit (effects are cumulative, not stacking)
1 - Bonus Feat, Flurry* @ -2 Attack
2 - Bonus Feat, Evasion
3 - Still Mind*, speed increase*
4 - Ki strike (magic), increased Unarmed damage
5 - Purity of Body*, Flurry @ -1 Attack, +1 AC
6 - Bonus Feat, speed increase
7 - Wholeness of Body*
8 - increased Unarmed damage
9 - Improved Evasion, speed increase, Flurry @ no penalty
10 - Ki strike (lawful), +2 AC
11 - Diamond Body*, Greater Flurry*
12 - Abundant Step*, increased unarmed damage, speed increase
13 - Diamond Soul
15 - Quivering Palm*, +3 AC, speed increase
16 - Ki strike (adamantine), increased unarmed damage
Alignment: any lawful
Hit Die: d8
Skill Points: 4 + Int mod
Skill List: Balance, Concentration, Diplomacy, Hide, Jump, Listen, Move Silently, Perform, Spot, Swim, and Tumble
Saves: Good/Good/Good (the only class that has such)
BAB: 3/4 (same as Rogue/Cleric/Bard)
Level 16 is kinda blah. Not giving up much to do 1 level of M/C.
Level 15 is quite the level. Make sure you're getting a lot of your M/C to give this stuff up.
Level 14 is the only useless Monk level on the chart (although 18 is pretty bad too, but I stopped at 16). If you give up 2 Monk levels, seriously consider giving up the third if you can get something worth it. You'll still hit 15 with the next level cap raise if you do.
Level 13 is some big Spell Resistance. For non-Drow, it should be hard to give up. Minimum SR 23 (higher with each monk level) is useful.
Level 12 and down are all some of the "basic" Monk stuff. You're losing your Monk-ness if you M/C yourself out of these. If you're giving up more than 4 levels of Monk, you might consider "splashing" monk into the other class instead.
As for the Bonus Feats, these are something we're going to have to "wait and see" about. Each "Bonus Feat" on the Monk chart is really just an option where you get to pick from 1 of 2 feats. However, of the 3 feats * 2 options each = 6 feats that comprise these lists, NONE of them made it to DDO. I personally expect to see feats such as Dodge, Combat Expertise, Mobility, and Spring Attack show up either on the lists or in place of the selection entirely.
Due to a Monk's "any lawful" alignment requirement and the Barbarian's "any non-lawful", a Monk cannot multiclass with Barbarian.
Due to a Monk's "any lawful" alignment requirement and the Bard's "any non-lawful", a Monk cannot multiclass with Bard.
The cleric class in general is not a popular choice to multiclass to. It offers borderline useless spell points and buffs in the endgame scenario, and wand usage is generally better achieved on a combat class through Paladin or Ranger. If you're willing to invest more than 1 or 2 levels, buffs can get useful, but the best Cleric buff in the books, Divine Power, you wouldn't get until level 7 Cleric. That's a little more multiclassing than I'd be willing to bite off.
For a melee fighting character, Fighter feats are always good. With 2 levels you get the +1 str enhancement too. Not a bad deal. The martial weapon proficies are a plus, too (though not as much for Elves).
I'm guessing Paladin is going to be the single most popular Monk multiclass in DDO. By giving up 3 levels of Monk, you get Cha to Saves, martial weapon proficiencies, healing wand usage, +2 AC, +2 saves, and Fear Immunity. A 1 level dip here is also a good option if you want to preserve Quivering Palm.
The second most popular Monk multiclass. 2 levels of Ranger has a lot to offer. Dex class enhancements, healing wand usage without the "good" alignment requirement, 2 good base saves, Two Weapon Fighting (stacks with Flurry), Strength bonus to bow damage, and martial weapons proficiency. Not a bad one-level investment either if you want to keep Quivering Palm.
Aside from UMD, I don't really see any reason to take Rogue levels unless you want to be a trapsmith. And at 4+ skill points per level with no skill synergy, it's like doing trapsmithing on a Barbarian. Intelligence is usually a dump stat on a Monk due to their need for good Str, Dex, Con, and Wis.
I personally don't see much any value multiclassing to Sorcerer. Like intelligence, Cha is usually a dump stat on a Monk.
Wizard has a significantly greater appeal to me than Sorcerer. Extend becomes a freebie, and 1 level of wizard gets you 2 minute shield spells for a free +4 AC. Monks can't use real shields so this bonus will always stack. Only downside it that it's dispellable. 3 levels of wizard gets you 6 minute Shield and Blur buffs. For an AC/Dex based monk, this is certainly a viable option.
*Flurry - when unarmed or using monk weapons, allows the Monk an extra attack.
*Still Mind - +2 saving throws vs enchantments
*Speed increase - A set increase in PnP that doesn't translate well to DDO. Each increase is the same diffence as Barbarian non-boosted speed increase.
*Purity of Body - Immune to all non-magical and non-supernatural diseases
*Wholeness of Body - Similar to a paladin's Lay on Hands, just not quite as good.
*Diamond Body - Immune to all poisons
*Greater Flurry - when unarmed or using monk weapons, the Monk's Flurry grants a second extra attack
*Abundant Step - similar to Dimension Door. See this post for Eladrin's comment on it.
*Diamond Soul - Spell Resistance 10 + Monk Level
*Quivering Palm - I dare not speculate how this will translate to DDO. Essenstially, a save-or-die melee attack (Will save based).